IOWA ACADEMY OF SCIENCES. 87 



McGee, in his great storehouse of facts regarding the drift 

 sheets of Iowa, mentions several points in the southern portion 

 of the state where there are more or less clear evidences of the 

 presence of two drift sheets. The Albia exposure, judg- 

 ing from the figure given,* represents the Kansan-loess con- 

 tact. Presumably the Durham exposuref is to be referred to 

 the same horizon. 



The Afton-Thayer exposures were visited by McGee and 

 Chamberlin in company, and the evidence of an interglacial 

 interval here, in connection with the facts derived from a study 

 of other portions of the Mississippi valley, was considered suf- 

 ficient to warrant the reference of the beds to two distinct 

 periods of glaciation. With a wise conservatism the two 

 periods were assumed to be the same as had been demonstrated 

 in northeastern Iowa, and accordingly in the nomenclature 

 eventually projDOsed by Chamberlin, I the upper drift at Afton 

 was considered to be the lowan, and the lower the Kansan. 

 The Aftonian beds proper were considered to represent the 

 interval between the Kansan and the lowan. It is important 

 to note that in the original paper by Chamberlin the term 

 Aftonian was not applied to the gravels which form so conspic- 

 uous a feature of the Afton-Thayer sections. These were con- 

 sidered to represent rather, kame-like accumulations upon the 

 surface of the older drift sheet. This distinction has not been 

 always clearly observed. 



The Afton-Thayer outcrops are for many reasons the most 

 important of those bearing on the question of an interglacial 

 interval in southwestern Iowa and will be described in some 

 detail. Preliminary to this it is desired to examine briefly 

 what sort of evidence may properly -be required to establish 

 the presence of two drift sheets. An excellent discussion of 

 the criteria for distinguishing between drift sheets has been 

 given by Salisbury§. At this point it is intended merely to 

 indicate certain of these criteria found to be of value in the 

 Iowa work, and to discuss briefly the importance which may be 

 legitimately attached to them. 



* Pleistoceae History, N. E. Iowa, Eleveoth Ann. Rep. D. S. Qeol. 8ur., p. 493. 

 + Op, Git., p. 494, pi. 111. 



* Great Ice Age(Gelkle), pp. 773-774, 1894; Jour. Geol., Vol. Ill, pp. 370-277. 1895. 

 § Jour. Geol., Vol. I, p. 61. 



